Chemical sensing is likely the most primordial sensory modality that emerged in the evolution of life. Without chemical sensing life on earth would probably not exist. It is used for detecting nutrients, avoiding threats, finding mating partners and various forms of communication and social interaction between animals.
The advent of artificial sensors has created a myriad of problems in the areas of chemical detection and identification with applications in food quality and pollution control, chemical threat detection, health monitoring, robot control and even odour and taste synthesis. Efficient algorithms are needed to address many challenges of chemical sensing in these areas, including (but not limited to) sensitivity levels, sensor drift, concentration invariance of analyte identity and complex mixtures.
As an example, biological pathogens, including biological threat agents, are living organisms that reproduce and sustain a population, which amplify, grow and re-infect, thereby resulting in an epidemic situation. The biological pathogens represent an extremely diverse range of microorganisms, which have no seemingly common attributes other than infecting the human and animal populations. The problem is therefore to detect and identify them at the earliest stage of invasion and at the lowest concentration.
Prior to DNA sequencing, the highest resolution techniques provided only protein and peptide-level structures as targets of analysis and assays. Many of the well-established protocols called for the examination of the size and shape of the pathogens along with the examination of the expressed proteins through biochemical and immunochemical assays. Advances in DNA sequencing technology have made it possible for scientists all over the world to sequence complete microbial genomes rapidly and efficiently. Access to the DNA sequences of entire microbial genomes has recently offered new opportunities to analyse and understand pathogens at the molecular level. Modern DNA sequencing techniques are able to detect pathogens in biological tissues and study variations in gene expression in response to the pathogenic invasion. These responses help in designing novel approaches for microbial pathogen detection and drug development. Identification of certain microbial pathogens as etiologic agents responsible for chronic diseases is leading to new treatments and prevention strategies for these diseases.
Majority of the modern chemical sensors used in pathogen detection are based upon the sequence-based recognition of DNA, structural recognition of pathogens or pathogen biomarkers, or cell-based function. However, the selection of the pathogen biomarkers introduces a serious challenge in the development of the sensors for detection of the biological pathogens. This is because most of the pathogen biomarkers have low selectivity and can distinguish between general classes of microorganisms, but are not able to identify the specific species or strain of organism. For example, calcium dipicolinate is a unique component of endospores. Dipicolinic acid can therefore be used to indicate the presence of endospores, but it cannot be able to distinguish between very dangerous Bacillus anthracis spores and other non-toxic Bacillus spores. The presence of the DNA as an additional indicator will be able to determine that the unknown material is biological in nature but will not be able to identify its source (unless extensive sequence-based analysis is used). Also cell metabolites are generally common to many different cell types and therefore extremely difficult to use for discrimination between specific microorganisms. In view of the above, there is a long-felt need for new methods and devices to detect and identify biological pathogens.
The use of the ultrasensitive and highly selective microelectronic sensors for the biological pathogen detection is the area that has not been developed yet. The reasons for that are many. Sensor arrays that detect multiple pathogen biomarkers produce a large number of false alarms because of their low selectivity. The concept of sensor arrays has been successfully used in the field of vapour analysis. In this approach each particular sensor of the sensor array was designed to respond to different properties of the vapours, followed by statistical methods to specifically identify the particular vapour from the fingerprint of the generated response from all the sensors of the array. However, since each pathogen species carries with it a unique DNA or RNA signature that differentiate it from other organisms, such approach cannot be effectively used for pathogen detection. In other words, each sensor of the array responds to different properties (biomarkers) of a pathogen. Therefore, such approach would require a well-characterised and already identified background signal to determine the fingerprints that would constitute a positive signal.
The ideal solution for a real-time sensing would be any specific response of a biological organism that results in instantaneous, specific and repeatable identification. However, as noted above, there are considerable technological and practical difficulties in the development of sensors that provide a real-time response for all three of these criteria. Immuno-assay techniques might give a similar specific analysis. However, their drawback, other than the long response time, is the requirement for special chemical consumables that add considerably to the logistic burden and costs. These can increase operational costs by hundreds of dollars per hour.
Optical technologies intrinsically result in real-time (bio)chemical detection. Sensors based on these technologies have been available to military and civil defence for quite some time. However, the common drawback of the optical sensors is low specificity. The sensors mostly offer a generic detection capability at best, since the optical similarity of the target particles with benign, naturally occurring backgrounds makes them difficult to distinguish. There are the some of the currently employed bio-agent detections strategies. Most represent a compromise between specificity, speed and cost.
Quantitative Polymerase Chain Reaction (qPCR) is capable of amplification and detection of a DNA sample from a single bio-agent cell within 30 minutes. Knowing the pathogen nucleic acid sequence makes it possible to construct oligos for pathogen detection. These oligos are at the basis of many highly specific analytical tests now on the market.
Microarray-based detection can combine powerful nucleic acid amplification strategies with the massive screening capability of microarray technology, resulting in a high level of sensitivity, specificity, and throughput. In addition to the previously mentioned caveats, the cost and organizational complexity of performing a large number of PCR reactions for downstream microarray applications render this option feasible but unattractive. This limitation has severely reduced the utility of this technique and impeded the continued development of downstream applications.
To sum up, the problem of accurate and reliable identification of pathogenic agents and their corresponding diseases is the weakest point in biological agent detection capability today. There is intense research for new molecular detection technologies that could be used for very accurate detection of pathogens that would be a concern to first responders. These include the need for ultrasensitive and highly selective sensors for biological pathogens detection in environmental, forensic and military applications. The benefits of specific (accurate) detection include saving millions of dollars annually by reducing disruption of the workforce and the national economy and improving delivery of correct protective countermeasures.
All said above regarding detection of biological pathogens also relate to the detection of other chemical and biological compounds, which may present threat or have medical reasons to be detected. The examples are many and may include explosives, toxins, DNA, proteins etc.
Surface acoustic-wave (SAW) sensors play an important role in many fields of chemical and biomolecular sensing. In general, a surface acoustic wave is an acoustic wave that propagates along the surface of a certain (piezoelectric) material. It is generated by interdigitated transducer (IDT) electrodes (or “fingers”), which are special periodic metallic bars deposited on a piezoelectric material. When any sinusoidal wave having a period equal to the period of the IDT electrodes is applied, mechanical vibration occurs beneath the IDT electrodes, thereby generating an acoustic wave, which is perpendicular to the geometry of the IDT bars. This acoustic wave propagates on the surface of the piezoelectric material away from the IDT electrodes in both directions.
The acoustic wave generated by the IDTs is localised in the surface region and penetrates the bulk piezoelectric material only to a wavelength deep region. That is why the SAW has a very high energy density at the surface, which gives the name “surface acoustic wave”. The SAW propagates in a piezoelectric material approximately 105 times slower than a regular electromagnetic wave. Consequently, the SAW wavelength in the piezoelectric material is 105 times smaller than the wavelength of an electromagnetic wave, making the SAW-based sensor a very compact device.
Fabrication of the SAW sensors requires either deposition or etching of the metallic IDTs on a piezoelectric material, and it uses the CMOS process technology, which allows a large scale manufacture.
The factors that can affect the piezoelectric material surface condition include pressure, temperature, humidity and mass loading. Accordingly, SAW sensors can be used as pressure, temperature, humidity sensors, and as sensors capable of detecting mass changes or electric field alterations at the surface. A MEMS-CMOS technology facilitates the integration of the SAW sensors and their data processing circuits. If a chemical or (bio)molecular layer sensitive to a certain chemical or biological target molecule is deposited at the delay line area of the SAW sensor, it allows this specific chemical or biological target molecule or analyte to react with the sensitive layer and consequently, to be bound at the delay line area. As a result, a mass change and/or electric field change is normally observed and the density of the target chemical or biological molecule (analyte) can be detected and further correlated to its concentration. Thus, the SAW sensors can be used as (bio)chemical molecular sensing devices, which is the subject of the present application.
Specially designed SAW sensors can also be used in a passive mode without need for batteries. An RFID antenna can be added to the input IDT electrode and the signal received by the antenna can then stimulate the SAW used for sensing as mentioned before. These are the zero-power SAW sensors which uses the RFID tag. The ultrahigh sensitivity, compact nature, ease of fabrication and wireless operation make these sensors very attractive for (bio)chemical detection and biomolecular diagnostics.
Penza et al (1998) (M. Penza, E. Milella, V. I. Anisimkin, “Monitoring of NH3 gas by LB polypyrrole-based SAW sensor”, Sensors and Actuators B: Chemical, 1998, 47(1-3), p. 218) described a SAW-based sensor that shows high sensitivity and selectivity towards gases, such as NH3, CO, CH4, H2 and O2 at room temperature by depositing polypyrrole films on the SAW surface as gas absorbent layers.
Lim et al (2011) (C. Lim, W. Wang, S. Yang, “Development of SAW-based multi-gas sensor for simultaneous detection of CO2 and NO2”, Sensors and Actuators B: Chemical, 2011, 154(1), p. 9) disclosed a reflective delay line SAW sensor that can measure CO2, NO2 and temperature simultaneously. By taking advantage of a zero-power technology in the SAW sensor, the sensor is operated without a battery, and the sensing of NO2, CO2 and temperature can be done simultaneously.
Raj et al (2013) (V. B. Raj, H. Singh, A. T. Nimal, “Oxide thin films (ZnO, TeO2, SnO2 and TiO2) based surface acoustic wave (SAW) E-nose for the detection of chemical warfare agents”, Sensors and Actuators B: Chemical, 2013, 178, p. 636) suggested sensing of chemical warfare agents using the SAW-based sensors with ZnO, TeO2, SnO2 and TiO2 deposited for the detection of dimethyl methylphosphonate, dibutyl sulfide, chloroethyl phenyl sulfide and diethyl chlorophosphate, respectively.
Cai et al (2105) (H. L. Cai, Y. Yang, X. Chen, “A third-order mode high frequency biosensor with atomic resolution”, Biosensors and Bioelectronics, 2015, 71, p. 261) described the SAW-based sensor used to detect DNA sequences and cells. The probe DNA and target DNA were attached to the surface and the resulting frequency change of the SAW resonator was measured.
Zhang et al (2015) (F. Zhang, S. Li, K. Cao, “A microfluidic love-wave biosensing device for PSA detection based on an aptamer beacon probe”, Sensors, 2015, 15(6), p. 13839) disclosed a prostate specific antigen (PSA) sensor. In this sensor, lithium tantalate (LiTaO3) with aluminium IDTs were coated with a wave guiding layer of silica, followed by deposition of a gold sensing layer for PSA attachment. Subsequently, a microfluidic channel was fabricated using PDMS to ensure that liquid can flow between the IDTs.